Fifteen minutes after Sara Bozorg, a doctor at Massachusetts Eye and Ear Infirmary in Boston, finished the Boston Marathon on Monday, she heard the first loud blast. Then she heard the second one, and spied the smoke rising.

Police quickly descended around the finish line and, with cell phones mostly not working, Bozorg and her boyfriend repaired to his home, turned on the news, and logged onto Facebook, where Bozorg posted that she was OK and that her phone was down.

"I have been following my friend's Facebook [account] who is near the scene and she is updating everyone before it even gets to the news," Bozorg said by e-mail on Monday night.

Indeed, as word spread of the blasts on Monday afternoon, social media seemed shaped by every aspect of the response, from runners giving their accounts of the race-turned-nightmare on Facebook, to authorities using Twitter to give instant updates, to The Boston Globe temporarily converting its homepage to a live blog that pulled in Tweets from Boston authorities, news outlets, and ordinary citizens.

The blasts have left three dead and have injured scores more, according to news reports.

Terrorism experts said that social media helped people in Boston and beyond determine their next steps after hearing about the explosions.

"Authorities have recognized that one the first places people go in events like this is to social media, to see what the crowd is saying about what to do next," said Bill Braniff, Executive Director of the National Consortium for the Study of Terrorism and Response to Terrorism. "And today authorities went to Twitter and directed them to traditional media environments where authorities can present a clear calm picture of what to do next."

"We know from crisis communication research that people typically search for corroborating information before they take a corrective action—their TV tells them there's a tornado brewing and they talk to relatives and neighbors. And now they look at Twitter."

On Monday, information about the blasts spread more quickly on social media than through traditional news outlets. "I was first notified to the event by my daughter, who was on Twitter, and that was before it came out on CNN," said Anthony C. Roman, president of a security consulting firm in New York called Roman and Associates.

That way of instantly spreading information far beyond the affected area also builds an online community around such events that can cross state and national borders. On Monday afternoon, #BostonMarathon quickly became a trending hashtag on Twitter.

"If I'm here in Washington, D.C., and I'm on Twitter and can demonstrate my empathy, it helps create this idea of resolve or community solidarity with people who are there on the ground in a way that uni-dimensional media doesn't do," said Braniff. "Online, I can express outrage or sympathy."

"I get a greater sense of unity—the we is a much bigger we," he said.

Terrorism experts said the proliferation of photos and video on the Web through social media might also help authorities identify the perpetrators of the attack.

"All the media provides a tremendous asset for the forensic evaluation of the explosion event," said Roman. "Authorities can start examining the pictures and tapes looking for individuals near the receptacles where the bombs were found and individuals not fitting the profile of the general spectator can be identified."

Libby Hemphill, an Illinois Institute of Technology professor who studies social media, said she's noticed big differences in how people use social media to react to national disasters and human-made ones like the Boston bombings.

Whereas during a natural disaster like Sandy, people never stopped documenting the aftermath in pictures and video, such documentation is already winding down after the Boston Marathon.

"Right after the explosions happened, we saw a lot of photographs, but that's dropped off now," she said. "People have stopped documenting and now are trying to make sense of what happened."